The present invention relates to flame detection and, more specifically in flame detection, to techniques involving analysis of radiation intensity variations for distinguishing flame radiation from interfering radiation.
In flame-detection techniques of interest, a radiation sensor receives radiation whose flicker characteristics in a very low frequency range are used to distinguish between interfering radiation and radiation originating from a flame. Simple means for delimiting the frequency range or band include radiation-input filters and frequency-selective sensor-signal amplifiers, in both cases for realizing a predetermined passband, e.g., from 5 to 25 Hz. But even if the passband is optimally chosen for the detection of flame flicker, malfunctioning and false indications are relatively frequent, as it is quite common for unanticipated intensity variations of ambient radiation to lie in the passband. Such intensity variations can be caused, e.g., by shading or reflections by vibrating or slowly moving objects, by reflections of sunlight from water surfaces, or by flickering or unsteady light sources.
U.S. Pat. No. 3,739,365 discloses a method of the aforementioned type in which the susceptibility to interfering light is reduced by use of two types of sensors with different spectral sensitivities, and forming of the difference between the two sensor output signals in a limited low-frequency range.
In practice, it has been found that the susceptibility to extraneous radiation sources, and thus the probability of false alarms remain relatively high because interfering radiation may well appear in the critical frequency range. For this reason, the critical frequency range in state-of-the-art flame detectors consists of just a few narrow frequency bands. For example, U.S. Pat. No. 4,280,058 discloses evaluation, for alarm, of emissions in a wavelength range of approximately 4.4 .mu.m, i.e., in a range which is characteristic of carbon-dioxide combustion. But still, this does not prevent interfering radiation in this wavelength range from triggering a false alarm.
Sought are reliability in flame detection, elimination of interfering radiation, minimization of false alarms, and broad applicability.